Recent advances in the field of superconductors have particularly involved certain oxide ceramic compositions that exhibit superconductive properties at temperatures approaching liquid nitrogen temperature (77.degree. K.) and higher. The first publication was by International Business Machines' Zurich Research Laboratory in Apr. 1986 for the oxide composition of barium, lanthanum and copper.
Subsequently scientists at the University of Houston discovered a better composition consisting of yttrium oxide, barium oxide, and copper oxide in the atomic proportions 1,2,3 (and thereby known as "1-2-3 composition") with the nominal formula YBa.sub.2 Cu.sub.3 O.sub.7-X. More broadly, an orthorhombic pervoskite crystal structure has been recognized as the basis for superconductive oxides. The problem of preparing shapes and samples of such oxide materials immediately became apparent, and plasma spraying of coatings was recognized as a viable method. There have been numerous publications pertaining to these developments; typical references are as follows: "Superconductor Research Pace Quickens" G. Fisher and M. Schober, Ceramic Bul. 66, 1087 (1987); "Thermal Spraying Superconducting Oxide Coatings", J. P. Kirkland et al, Advanced Ceramic Materials 2, No. 3B Special Issue, 401 (1987).
However, realization of full potential for superconductive properties has remained elusive. The superconductivity has been incomplete and lacking in reproducibility. The problem of sufficient and reliable superconductivity, for example in plasma sprayed coatings, has been traced broadly to the quality of the superconductive powder utilized, particularly the stoichiometry of constituents including oxygen and the presence of contaminants. Details are not very well understood. Superconductive ceramics such as the 1-2-3 type are particularly susceptible to reduction of the oxide. A further problem is that superconductivity is detrimentally sensitive to effects in grain boundaries which are inherent from the processing. Subsequent annealing of the material in oxygen helps, but with only some improvement.
Since there remains a lack of understanding of the superconductivity phenomenon in ceramics, the materials depend on and are still best defined by method of production. One common method of making powder involves laborious steps of milling, grinding, calcining, sintering, annealing, and crushing. These difficulties have resulted in a practical inability to manufacture such materials in large quantities. Also, sintering methods do not effect complete homogeneity, which could result in less than optimal superconductive properties.
Several currently known solution chemistry techniques are being explored, but do not appear to have resulted in the production of large quantities of superconductive powder. Additionally, powders made by solution techniques are generally in the 1-2 micron range. An example of a chemical processing method for producing a powder is disclosed in U.S. Pat. No. 4,654,075 (Cipollini).
Thermal sprayable multi-component ceramic powders can be made by the spray drying process such as is described in U.S. Pat. No. 3,617,358 (Dittrich). While this method enables production of large quantities of powders, the constituents of such powders are not alloyed, and there have been difficulties in obtaining a superconductive coating. Coatings deposited from spray dried composite powders are inhomogeneous and require extended heat treatments at 950.degree.0 C. that often degrade the substrate and also do not produce consistently reproducible superconducting coatings. It is also known to further process the spray dried powder by passing it through an arc plasma gun as disclosed in U.S. Pat. No. 3,974,245 (Cheney et al). However it has been found that, in the case of superconducting compositions, this process step results in powders that are deficient in oxygen and, also, cation ratios are substantially altered due to selected evaporation. The spray dried superconducting ceramics further present severe practical difficulties in plasma processing due to powder port and nozzle build up and result in very low yields and a poor quality powder.
Similarly it is known to spheroidize granular refractory oxides. U.S. Pat. No. 3,278,655 (Barr) teaches such spheroidizing of uranium oxide with an additive in a combustion flame. U.S. Pat. No. 3,171,714 (Jones et al) concerns spheroidizing granular plutonium oxide in an oxygen enriched induction plasma. Problems of retaining cation ratios in the high temperature plasma, and even in a combustion flame, are similar to those of a DC plasma. A related problem is further change in cation ratio during the subsequent thermal spray coating process.
Therefore an object of the present invention is to provide a novel method of producing superconductive powder. A further object is to provide an improved superconductive powder. An additional object is to produce thermal spray powders for the deposition of improved superconductive coatings Another object of the invention is to enable large scale manufacture of the said powder. Yet a further object of the invention is to prepare powders having a desired chemistry and size and with sphericity for good flowability. Yet another object is to obtain a powder with the desirable superconductive orthorhombic crystal structure.